Pressure measuring devices serve for registering pressures, especially absolute pressures, relative pressures and pressure differences, and are used in industrial measurements technology.
In pressure measuring technology, so called semiconductor pressure sensors are a welcome option. Semiconductor sensors are today regularly produced based on silicon, e.g. by applying Silicon-On-Insulator (SOI) technology. Such are embodied in the form of a pressure sensor chip, which typically involves a support and a measuring membrane, or diaphragm, arranged on the support. In measurement operation, a first pressure is supplied to a first side of the measuring membrane, or diaphragm.
For registering pressure differences, a second side of the measuring membrane, or diaphragm, lying opposite to the first side is supplied with a second pressure. The pressure difference existing between the first and second pressures effects a deflection of the measuring membrane, or diaphragm, dependent on the pressure difference to be measured.
For registering relative pressures, the second side of the measuring membrane, or diaphragm, is supplied a reference pressure, e.g. ambient pressure. The existing pressure difference between the first pressure and the reference pressure effects a deflection of the measuring membrane, or diaphragm, dependent on the relative pressure to be measured.
For registering absolute pressures, there is usually provided under the measuring membrane, or diaphragm, on its second side facing away from the first side, a sealed, evacuated chamber. Therewith, the first pressure acting on the first side of the measuring membrane, or diaphragm, effects a deflection of the measuring membrane, or diaphragm, dependent on the absolute pressure to be measured.
The resulting deflection of the measuring membrane, or diaphragm, is, in all three cases, registered via sensor elements, e.g. piezoresistive resistors, arranged on the measuring membrane, or diaphragm, and converted into an electrical output signal, that then is available for additional processing and/or evaluation.
Semiconductor pressure sensors are very sensitive and are therefore applied in a housing, via which supply of the respective pressures, output of measurement results and mounting of the pressure measuring device at the measuring location occurs.
In such case, the semiconductor pressure sensor is mounted, for example, on a pedestal located in the housing, in such a manner that a first side of the measuring membrane, or diaphragm, facing away from the pedestal, faces into a first measuring chamber located in the housing. This first measuring chamber receives the first pressure. In the case of difference, or relative, pressure sensors, supplementally, the second pressure, or the reference pressure, is supplied to the second side of the measuring membrane, or diaphragm, via a bore extending in the interior of the pedestal. The bore opens into a second measuring chamber enclosed by the measuring membrane, or diaphragm, support, under the measuring membrane, or diaphragm. In the case of absolute pressure measuring devices, the chamber located under the measuring membrane, or diaphragm, is sealed and evacuated. The supply of the first and second pressures occurs, for example, via pressure transfer means integrated into the housing, or connected in front thereof, and filled with a pressure transmitting liquid. The supply of the reference pressure occurs, for example, via a reference pressure supply integrated into the housing.
The pedestal is, for example, a cylindrical protrusion, which is embodied as an integral component of the housing or as a separate component secured in the housing.
In order to assure a sufficiently high mechanical stability, housing and pedestal are composed of a mechanically stable material, especially metal.
Pedestal and semiconductor pressure sensor are composed therewith unavoidably of different materials, which have very different physical properties, especially different coefficients of thermal expansion. Due to the mechanical connection between the pedestal and the semiconductor pressure sensor, consequently, mechanical stresses can occur, which affect the transfer behavior of the measuring membrane, or diaphragm, and therewith the achievable accuracy of measurement. Reproducibility of measurements worsens. This is true especially in the case of temperature dependent stresses.
For reducing the arising stresses, usually an intermediate piece is inserted between the pedestal and the semiconductor pressure sensor. The intermediate piece is composed of the same material as the semiconductor pressure sensor. Also then, however, there still occurs, because of the different coefficients of thermal expansion of pedestal and intermediate piece, especially temperature dependent, mechanical distortions, which affects the transfer behavior of the measuring membrane, or diaphragm.
In German Patent DE 34 36 440, a solution for this problem is described, which enables a reducing of the disadvantageous effects of the mechanical stresses. Such a measuring unit is shown in FIG. 1. It comprises a semiconductor pressure sensor 1 having a measuring membrane, or diaphragm, 5 carried by a support 3. The support 3 is mounted on an intermediate piece 7 that is arranged on a metal pedestal 9. A bore is provided, which leads through the pedestal 9 and the intermediate piece 7 and into a measuring chamber enclosed under the measuring membrane, or diaphragm, 5. The intermediate piece 7 is circularly disk shaped and has an outer diameter matched to the outer diameter of the semiconductor pressure sensor 1. The pedestal 9 is hollow cylindrically embodied and includes a markedly smaller outer diameter. For reducing mechanical stresses, there is provided on the underside of the intermediate piece 7 toward the pedestal 9 a ring-shaped groove 11, which borders directly on the pedestal 9. The groove 11 serves to absorb mechanical stresses caused by the connection between pedestal 9 and intermediate piece 7 and to prevent the mechanical stresses from reaching the measuring membrane, or diaphragm, 5.
This measuring device has, however, the disadvantage, that the mechanical stability of the measuring unit limited by the bond strength of the connection between the pedestal 9 and the intermediate piece 7.
Especially, in the case of pressure difference measuring systems, the maximum pressure, which can be supplied through the pedestal to the measuring membrane, or diaphragm, is limited.
From safety reasons, this connection must assure that the semiconductor pressure sensor remains on the pedestal, when a positive pressure is supplied via the pedestal.
The bond strength of the connection between the pedestal and the intermediate support could theoretically be improved by enlarging the available connecting area between the pedestal and the intermediate piece. This leads, however, to the fact that the wall thickness between the bore and the annular groove rises, and the position of the annular groove is shifted radially outwardly. In this way, the annular groove loses, however, its effect as regards mechanical decoupling. This effect is especially marked in the case of pressure measuring devices designed for higher pressure measuring ranges, since these typically have semiconductor pressure sensors with measuring membranes, or diaphragms, of smaller diameter.
Alternatively or supplementally, the thickness of the intermediate piece could be increased and/or the depth of the groove made greater. A deeper groove leads, however, to reduced fracture safety. Increasing the thickness of the intermediate piece leads, in turn, to an increasing of the manufacturing costs, especially when the intermediate piece is made of silicon.